The present invention relates generally to casing used for packaging food products such as sausage and the like. More particularly it is concerned with a new and improved casing substrate and reinforced casing made therefrom, as well as with a new and improved method of producing such substrates and casings.
Heretofore it has been the practice to make reinforced films, tubing, casings or skins for food products and the like by sequential and repeated treatment of fibrous base papers or webs with a cellulose coating material, such as cellulose xanthate, hereinafter referred to as viscose. That treatment includes the preparation of a bonded casing substrate having substantial caustic stability followed by the final casing-forming operations. The caustic stable substrate is prepared by bonding a preformed and dried paper or fibrous base web with a dilute viscose solution followed by the steps of drying, regenerating the cellulose, washing and redrying. This initial bonding operation using the dilute viscose solution is required in order to impart sufficient caustic resistance to the bonded substrate to retain its structural integrity during the final casing-forming operations where treatment with a more concentrated viscose solution is carried out under highly alkaline conditions. The bonded substrate must also retain its porous, absorbent characteristics in order to permit complete impregnation and encasement by the concentrated viscose solution. The bonded substrate then undergoes the casing-forming operation that includes the steps of forming the substrate into a cylindrical tube, impregnating and encasing the substrate tube with a highly caustic viscose solution, regenerating the impregnate with acid, washing to remove excess acid and viscose and drying of the final fiber reinforced leakproof cellulosic film, hereinafter referred to as the casing or skin. This process is set forth in greater detail in the Underwood U.S. Pat. No. 3,135,613 entitled "Impregnated Paper Webs and Method of Making Sausage Casing Thereof", thus clarifying the sequential evolution of the base web to the bonded substrate and then to the reinforced casing.
The tubular casings produced in the manner set forth possess enough strength and burst resistance to be particularly well suited for enclosing meat and other food products such as fish or vegetables that are injected into the interior of the tubes under pressure. They thereby provide firm uniform enclosures for well known products such as sausage, bologna and the like as well as other food products. Modern processing equipment makes such products automatically but requires casing material that withstands high stuffing pressures without bursting and can be filled to a known and uniform diameter. The product may be automatically sliced and wrapped and must meet a stated meat content since any increase in the diameter of the casing over the required minimum necessarily results in a loss to the meat packer. Although casing substrates made according to the prior art have yielded casing of satisfactory strength and uniformity for the equipment utilized heretofore, modern machinery has required casings of improved strength characteristics.
Various patents subsequent to the aforementioned U.S. Nos. 3,135,613 have discussed the use of alternative materials for bonding the paper webs to provide appropriate casing substrates. In selecting bonding materials other than the commercially employed acid-regenerated viscose, it is important that the bonding materials meet both the processing and performance requirements of the food casings produced therefrom. In particular, the fibrous base web to which the bonding agent is to be applied must exhibit sufficient strength to withstand the stresses exerted during the coating operation. The amount of bonding agent cannot interfere with subsequent viscose penetration during the casing manufacturing process so that there is a loss of strength in the casing, or that there is a detrimental effect on the appearance of the casing. Also, the bonding agent should be one which will not cause the substrate to become discolored during exposure to the conditions of the casing forming process. In U.S. Pat. No. 3,484,256 to Chiu et al, it is suggested that the dilute viscose bonding treatment be replaced by the use of a bonding agent or mixture of a cationic thermosetting resin and a polyacrylamid resin. A bonding mixture of a cationic alkaline curing resin and carboxymethyl cellulose is disclosed in U.S. Pat. No. 3,468,696 to Conway as a substitute wet strength bonding treatment. The U.S. Pat. Nos. 3,640,734 and 3,640,735 to Oppenheimer et al teach the formation of substrates using insolublized poly (vinyl alcohol) or poly (vinyl alcohol)/poly (vinyl ester) copolymers such as poly (vinyl alcohol)/poly (vinyl acetate).
More recently U.S. Pat. Nos. 4,762,564 and 4,789,006 to Bridgeford et al have indicated that viscose casing films can be replaced by cellulose aminomethanate (hereinafter referred to as cellulose carbamate) films, thereby eliminating the toxic and noxious sulfur containing chemical species associated with viscose manufacture and use. However, U.S. Nos. 4,762,564 states that carbamate films have a lower tensile strength than viscose films. Since reinforced casings made from carbamate only exhibit strength characteristics essentially comparable to casings made from reinforced viscose, only the manufacturing advantages that avoid toxic materials have been emphasized. Unfortunately, Bridgeport apparently found that the substitution of carbamate films for viscose films lacked improved results regardless of the substrate employed. Recognizing this, U.S. Pat. Nos. 4,777,249 and the aforementioned U.S. 4,789,006 reported that cross linking of the carbamate casing with highly acidic solutions of glutaraldehyde or melamine formaldehyde was needed to improve the strength of the carbamate film. It has also been found, as reported herein, that for casing using a carbamate bonded fibrous substrate, the casing strength is significantly lower for carbamate casing than for viscose casing. Cross linking of the reinforced films at high concentrations of cross linking agents have not been effective to improve the properties of those films.
It has now been advantageously found that significant improvements can be achieved even with carbamate casings by improving the strength characteristics of the substrates used to reinforce the casing. This improvement can be obtained while retaining the non toxic advantages of carbamate film use relative to the manufacture and use of viscose.
Other advantages will be in part obvious and in part pointed out more in details hereinafter.
These and related advantages are obtained by providing a porous bonded fibrous substrate comprising a fibrous base web containing about 10% by weight or less of a particular bonding agent. The bonding agent is formulated by dissolving cellulose carbamate and an alkaline curing resin in a dilute caustic solution in such a manner that the ratio of carbamate to resin is greater than 1:1 by weight. The bonded substrate exhibits a porosity that is not significantly less than that of the base web, i.e., a porosity of at least about 300 liters/min as determined by TAPPI test method T25l-pm-75, a caustic tensile strength greater than 300 grams per 25 millimeters and is readily adapted to form reinforced casing films of improved burst strength. Casings made from the substrate can use the casing films employed heretofore or a carbamate-resin film formulation of the present invention.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties and relation of elements exemplified in the following detailed disclosure.